Relatively low-power (e.g., 10 dB above noise floor) auxiliary signals encoding digital information can be admixed together with composite video signals without being objectionably evident in television pictures generated from those composite video signals, if suitable restrictions on the digital signal format are observed. It is advantageous to use a suppressed, vestigial-sideband, amplitude-modulated (VSB AM) carrier of the same frequency as the VSB AM picture carrier, but in quadrature phasing therewith, for transmitting the digital data. This procedure permits the synchronous detection of the modulation of that quadrature carrier to recover the digital data. If the bandwidth of the receiver is sufficient to include the entire vestigial sideband, remnant composite video signal accompanying the digital data as an interfering signal will not have substantial energy in the baseband extending up to 0.75 MHz in frequency. It is around 0.75 MHz that the VSB AM video carrier begins the transition from being a double-sideband amplitude-modulated (DSB AM) carrier to being a single-sideband amplitude-modulated (SSB AM) carrier, and at lessened energy up to the 1.25 MHz frequency at which roll-off of the vestigial sideband is complete.
A. L. R. Limberg, C. B. Patel and T. Liu in their U.S. patent application Ser. No. 08/108,311 filed 20 Aug. 1993, entitled APPARATUS FOR PROCESSING MODIFIED NTSC TELEVISION SIGNALS, WITH DIGITAL SIGNALS BURIED THEREWITHIN, and incorporated by reference herein describe phase-shift-keying (PSK) modulation of a subcarrier of the VSB AM carrier that is in quadrature phasing with the VSB AM video carrier of the same frequency. The frequency of their subcarrier is an odd multiple of one-half scan line frequency, and it is phase-shift-keyed in accordance with serial-bit digital data supplied at a symbol rate that is a multiple of scan line frequency. Limberg et alii prefer transmitting frames of the modulated subcarrier twice, but in opposite phasing in each successive pair of consecutive frames of the NTSC television signal. Because of frame-averaging effects resulting from the limitations on the speeds of the response of the human visual system and the decay of electroluminescence of kinescope phosphors, such repetition of data in pairs of frames makes PSK subcarrier accompanying the composite video signal detected from the NTSC television signal less visible in images that are generated from the composite video signal for viewing on a screen. Such repetition of data in pairs of frames also provides a basis for using frame-comb filtering in a digital signal receiver to separate PSK subcarrier from the luminance portion of the composite video signal that describes static portions of successive television images. Limberg et alii prefer also repeating the modulation of the digital data in antiphase in contiguous pairs of adjoining scan lines of the NTSC television signal, providing a basis for using line-comb filtering in the digital signal receiver to separate PSK subcarrier from the chrominance portion of the composite video signal.
Limberg et alii describe a digital signal receiver in which the synchronous video detector for quadrature-phase VSB AM video carrier is followed by a cascade connection of a lowpass line-comb filter and a highpass frame-comb filter. The lowpass line-comb filter is for separating the frequency spectrum of a PSK subcarrier having a frequency that is an odd multiple of half-scan-line frequency from chrominance signal portions of the frequency spectrum of an NTSC signal, particularly of an NTSC signal that has been appropriately pre-filtered. The highpass frame-comb filter is for separating the frequency spectrum of a PSK subcarrier having a frequency that is an odd multiple of half-scan-line frequency from motion-free luminance signal portions of the frequency spectrum of an NTSC signal. Limberg et alii teach that the remnant spectrum of the NTSC signal in the response of the cascaded highpass comb filters can be viewed as the frequency spectrum of a jamming signal accompanying the PSK signal. Accordingly, the remnant spectrum of the NTSC signal in the response of the cascaded highpass comb filters can be discriminated against by synchronous symbol detection.
J. Yang describes binary phase-shift-keyed (BPSK) modulation of a suppressed carrier that is the same frequency as a video carrier and is in quadrature phasing therewith in his U.S. patent application Ser. No. 08/141,070, filed 26 Oct. 1993, entitled APPARATUS FOR PROCESSING NTSC TV SIGNALS HAVING DIGITAL SIGNALS ON QUADRATURE-PHASE VIDEO CARRIER and incorporated herein by reference. The suppressed carrier that is in quadrature phasing with the video carrier is phase-shift-keyed directly, without any subcarrier being used. Yang also advocates transmitting frames of the modulated subcarrier twice, but in opposite phasing in each successive pair of consecutive frames of the NTSC television signal, just as Limberg et alii do. Yang advocates the BPSK signals being constrained to about 2 MHz bandwidth, so as to avoid crosstalk into chroma in TV receivers that separate chroma from luma without recourse to comb filtering. Yang indicates a preference for passing the data to be transmitted through a pre-line-comb partial-response filter prior to its digital-to-analog conversion to an analog modulating signal for a balanced amplitude modulator. This is done to preserve the information contained therein when line-comb filtering is done in the digital signal receiver to separate PSK subcarrier from the luminance portion of the composite video signal. Line-comb filtering in the digital signal receiver converts the partial-response filtered binary digital signal to ternary digital signal, if the line-comb filtering is of the two-tap type, linearly combining signals differentially delayed by only the duration of one horizontal scan line of video signal. Line-comb filtering in the digital signal receiver converts the partial-response filtered binary digital signal to five-level digital signal, if the line-comb filtering is of the three-tap type, linearly combining signals differentially delayed by the duration of one horizontal scan line of video signal and by the duration of two horizontal scan lines of video signal. Therefore, multi-level symbol decision circuitry is required to recover bit-serial digital data transmitted by the BPSK from the comb filtering response.
U.S. patent application Ser. No. 08/179,618 filed 5 Jan. 1994 by J. Yang and A. L. R. Limberg, entitled "PRE-FRAME-COMB" AS WELL AS "PRE-LINE-COMB" PARTIAL-RESPONSE FILTERING OF BPSK BURIED IN A TV SIGNAL, describes a pre-frame-comb partial-response filter as well as pre-line-comb partial-response filtering being used at the digital signal transmitter for processing bit-serial data from which BPSK modulating signal is generated for the carrier in quadrature phasing with the video carrier. Line-comb filtering in the digital signal receiver converts the partial-response filtered binary digital signal to five-level digital signal, if the line-comb filtering is of the two-tap type, linearly combining signals differentially delayed by only the duration of one horizontal scan line of video signal. Line-comb filtering in the digital signal receiver converts the partial-response filtered binary digital signal to nine-level digital signal, if the line-comb filtering is of the three-tap type, linearly combining signals differentially delayed by the duration of one horizontal scan line of video signal and by the duration of two horizontal scan lines of video signal.
U.S. patent application Ser. No. 08/179,588 filed 5 Jan. 1994 by J. Yang and A. L. R. Limberg, and entitled APPARATUS FOR PROCESSING BPSK SIGNALS TRANSMITTED WITH NTSC TV ON QUADRATURE-PHASE VIDEO CARRIER, describes BPSK modulating signal for the carrier in quadrature phasing with the video carrier being generated directly from bit-serial data without any pre-comb-filter partial-response filtering. The same patent application describes digital signal receivers, which use a cascade connection of a highpass frame-comb filter and a highpass line-comb filter after the quadrature video detector to suppress interfering remnant luminance signal, which use plural-level symbol decision circuitry for the comb filter response, and which use post-comb-filter partial-response filtering after the symbol decision circuitry for undoing the data alteration caused by the comb filtering.
Receivers for the Yang system are also described by T. V. Bolger in his U.S. patent application Ser. No. 08/141,071, filed 26 Oct. 1993, entitled RECEIVER WITH OVERSAMPLING ANALOG-TO-DIGITAL CONVERSION FOR DIGITAL SIGNALS WITHIN TV SIGNALS, and incorporated herein by reference. These receivers digitize the reponse of a quadrature-phase video detector using an oversampling analog-to-digital converter. The digitized quadrature-phase video detector response is subjected to digital frame-comb and line-comb filtering to suppress remnant composite video signals; the comb filtering response is supplied to multi-level symbol decision circuitry to recover bit-serial digital data transmitted by the BPSK; and the bit-serial digital data is supplied to a decoder that corrects the digital information in the data using forward-error-correcting codes contained therein.
Receivers for the Yang system are also described by J. Yang, T. V. Bolger and A. L. R. Limberg in U.S. patent application Ser. No. 08/179,586 filed 5 Jan. 1994, entitled RECEIVER WITH SIGMA-DELTA ANALOG-TO-DIGITAL CONVERSION FOR DIGITAL SIGNALS BURIED IN TV SIGNALS, and incorporated herein by reference. These receivers digitize the response of a quadrature-phase video detector using an oversampling analog-to-digital converter of sigma-delta type. Preferably, the bit resolution of a basic multiple-bit-resolution flash converter is improved by using a sigma-delta procedure in which only a single bit of the basic multiple-bit-resolution ADC output signal is converted back to analog signal for feedback purposes during each oversampling step, as described by T. C. Leslie and B. Singh in their paper "An Improved Sigma-Delta Modulator Architecture", 1990 IEEE SYMPOSIUM ON CIRCUITS & SYSTEMS, 90 CH 2868-8900000-0372, pp. 372-375, incorporated herein by reference. The digitized quadrature-phase video detector response is subjected to digital frame-comb and line-comb filtering to suppress remnant composite video signals; the comb filtering response is supplied to multi-level symbol decision circuitry to recover bit-serial digital data transmitted by the BPSK; and the bit-serial digital data is supplied to a decoder that corrects the digital information in the data using forward-error-correcting codes contained therein.
The inventions described in the patent applications referred to above, like the inventions described herein, are assigned to Samsung Electronics Co., Ltd., pursuant to pre-existing employee agreement so to assign inventions made within the scope of employment. In these patent applications the bit-serial data used for generating the binary phase-shift-keying signal have been processed at the transmitter so that the data will survive comb filtering procedures, which are carried out in the digital signal receiver for suppressing the composite video signals accompanying the data and tending to act as a jamming signal. With regard to operation of the combined NTSC television and BPSK transmitter, partial-response filtering of the bit-serial data subsequently used for generating the binary phase-shift-keying signal is advocated by each of these U.S. patent applications, except for Ser. No. 08/108,311.
In all the digital signal receivers described in the patent applications referred to above, there is a concern to reduce the errors in the reproduced digital data that are attributable to multipath effects, commonly referred to as "ghosting". Such effects are well known to television engineers, occurring quite often in television pictures that have been broadcast over the air or have been transmitted by cable.
The signal to which the television receiver synchronizes is the strongest of the signals it receives, which is called the reference signal, and is usually the direct signal received over the shortest reception path. The multipath signals received over other paths are thus usually delayed with respect to the reference signal and appear as trailing ghost images. It is possible, however, that the direct or shortest path signal is not the signal to which the receiver synchronizes. When the receiver synchronizes to a reflected (longer path) signal, there will be a leading ghost image caused by the direct signal, or there will be a plurality of leading ghosts caused by the direct signal and other reflected signals of lesser delay than the reflected signal to which the receiver synchronizes. The parameters of the multipath signals--namely, the number of different-path responses, the relative amplitudes of the different-path responses, and the differential delay times between different ones of the different-path responses--vary from location to location and from channel to channel at a given location. These parameters may also be time-varying.
The visual effects of multipath distortion can be broadly classified in two categories: multiple images and distortion of the frequency response characteristic of the channel. Both effects occur due to the time and amplitude variations among the multipath signals arriving at the reception site. When the relative delays of the multipath signals with respect to the reference signal are sufficiently large, the visual effect is observed as multiple copies of the same image on the television display displaced horizontally from each other. These copies are sometimes referred to as "macroghosts" to distinguish them from "microghosts", which will be presently described. In the usual case in which the direct signal predominates and the receiver is synchronized to the direct signal, the ghost images are displaced to the right at varying position, intensity and polarity. These are known as trailing ghosts or "post-ghost" images. In the less frequently encountered case where the receiver synchronizes to a reflected signal, there will be one or more ghost images displaced to the left of the reference image. These are known as leading ghosts or "pre-ghost" images.
Multipath signals of relatively short delays with respect to the reference signal do not cause separately discernible copies of the predominant image, but do introduce distortion into the frequency response characteristic of the channel. The visual effect in this case is observed as increased or decreased sharpness of the image and in some cases loss of some image information. These short-delay, close-in or nearby ghosts are commonly caused by unterminated or incorrectly terminated radio-frequency transmission lines such as antenna lead-ins or cable television drop cables. In a cable television environment, it is possible to have multiple close-in ghosts caused by the reflections introduced by having several improperly terminated drop cables of varying lengths. Such multiple close-in ghosts are frequently referred to as "micro-ghosts".
Long multipath effects, or macroghosts, are typically reduced by cancelation schemes. Short multipath effects, or microghosts, are typically alleviated by waveform equalization, generally by peaking and/or group-delay compensation of the video frequency response.
Since the characteristics of a transmitted television signal are known a priori, it is possible, at least in theory, to utilize such characteristics in a system of ghost signal detection and cancelation. Nevertheless, various problems limit this approach. Instead, it has been found desirable to transmit repeatedly a reference signal situated, for example, in a section of the TV signal that is currently unused for video purposes and to utilize this reference signal for detection of ghost signals prior to arranging for the suppression of ghost signals. Typically, lines in the vertical blanking interval (VBI) are utilized. Such a signal is herein referred to as a Ghost Canceling Reference (GCR) signal; and a variety of different GCR signals have been described in patents and other technical publications.
Bessel pulse chirp signals are used in the GCR signal that is the de facto standard for television broadcasting in the United States of America. The distribution of energy in the Bessel pulse chirp signal has a flat frequency spectrum extending continuously across the video frequency band. The chirp starts at the lowest frequency and sweeps upward in frequency therefrom to the 4.1 MHz highest frequency. The chirps are inserted into the first halves of selected VBI lines, the 19.sup.th line of each field currently being preferred. The chirps, which are on +30 IRE pedestals, swing from -10 to +70 IRE and begin at a prescribed time after the trailing edges of the preceding horizontal synchronizing pulses. The chirp signals appear in an eight-field cycle in which the first, third, fifth and seventh fields have a polarity of color burst defined as being positive and the second, fourth, sixth and eighth fields have an opposite polarity of color burst defined as being negative. The initial lobe of a chirp signal ETP that appears in the first, third, sixth and eighth fields of an eight-field cycle swings upward from the +30 IRE pedestal to +70 IRE level. The initial lobe of a chirp signal ETR that appears in the second, fourth, fifth and seventh fields of the eight- field cycle swings downward from the +30 IRE pedestal to -10 IRE level and is the complement of the ETP chirp signal.
The strategy for eliminating ghosts in a television receiver relies on the transmitted GCR signal suffering the same multipath distortions as the rest of the television signal. Circuitry in the receiver can then examine the distorted GCR signal received and, with a priori knowledge of the distortion-free GCR signal, can configure an adaptive filter to cancel, or at least significantly attenuate, the multipath distortion. A GCR signal should not take up too much time in the VBI (preferably no more than one TV line), but should still contain sufficient information to permit circuitry in the receiver to analyze the multipath distortion and configure a compensating filter to cancel the distortion. The GCR signals are used in the television receiver for calculating the adjustable weighting coefficients of a ghost-cancelation filter through which the composite video signal from the video detector is passed to supply a response in which ghosts are suppressed. The weighting coefficients of this ghost-cancelation filter are adjusted so it has a filter characteristic complementary to that of the transmission medium giving rise to the ghosts. The GCR signals can be further used for calculating the adjustable weighting coefficients of an equalization filter connected in cascade with the ghost-cancelation filter, for providing an essentially flat frequency spectrum response (or other preferred frequency spectrum response) over the complete reception path through the transmitter Vestigial-sideband amplitude-modulator, the reception medium, the television receiver front-end and the cascaded ghost-cancelation and equalization filters.
In a digital signal receiver for receiving digital signals buried in conventional analog television signals, there are advantages to detecting the composite video signal modulating the amplitude of the VSB video carrier using an in-phase video detector, in addition to a quadrature-phase video detector for recovering digital information. The synchronizing pulses for the composite video signal contain a large amount of useful timing information, which can be used to define data frames, data rows, and approximate PSK symbol positions. This timing information can also be used for controlling frame-comb and line-comb filtering of the signal detected by the quadrature-phase video detector, in order to suppress interfering remnants of the composite video signal. These remnants are above the 0.75 MHz frequency where the VSB AM video carrier begins the transition from being a double-sideband amplitude-modulated (DSB AM) carrier to being a single-sideband amplitude-modulated (SSB AM) carrier, exhibiting increased energy up to the 1.25 MHz frequency at which roll-off of the vestigial sideband is complete. The GCR signals transmitted in the 19.sup.th scan line of each field provide information concerning a modulo-8 field (or half-frame count) that is useful in relating frames of data to each other. Since an in-phase video detector is advantageously included in a digital signal receiver, anyway, the GCR signals it detects from the 19.sup.th scan line of each field are available as a basis for calculating multipaths in the transmission channel.